CMake does not check for Allegro and enet presence

[stephanemagnenat]

... while it should ;-)

Also, it should check version, for instance al_premul_rgba is only available starting from 5.1.12, but 5.0.11 is the default in Ubuntu 16.04 LTS (but is available as PPA).

[Orpheon]

I fixed the title issues, see commit 63c9885 , I don't know how to check for versions though.
We all use allegro 5.2.

In general the cmake file is kind of terrible, I don't really know how to use cmake and this is more or less the bare minimum that works. If you know how to do this properly, it would be a great help.